1. Field of the Invention
The present invention relates to injection needles especially needles mounted in a needle hub fitting onto an injection device of the kind from which preset doses of a medicine from a cartridge accommodated in the device may be administered through the needle mounted on the device and exposing a back needle penetrating a closure membrane of the cartridge and a free injection part.
2. Description of Related Art
Such an injection device, by which doses may be set individually and on which a needle may be mounted and changed after having been used for injection of a medicine from a cartridge in the device, is a common tool for people who have to inject themselves one or more times a day as it is the case by people who are treated with growth hormone or by diabetics who have to inject themselves frequently to keep their blood sugar on an acceptable level.
To reduce the malaise by frequent pricking of the skin the trend has lead towards use of still thinner needles as it have shown to cause less pain to be pricked with a fine needle than with a coarse one. With finer needles other problems occur one of them being that e.g. some kinds of insulin has coarse crystals which tends to clog at the inlet to the needle and in this way be sieved from the liquid in which the crystals are suspended. This way the concentration of the insulin injected may differ from what the user think it is which may cause injection of a wrong dose.
These problems has according to WO 93/00948 been overcome by using needles mounted in special needle hubs which fit only on devices about which it is known that they will only contain insulin which may flow freely through a thin needle defined as a G30 needle. This is obtained by making devices for which it is guaranteed that they will only contain insulin having a grain size less than 15 .mu.m and provide the devices with needle receiving pieces onto which the hubs with the fine needles fits. It may be noticed that thicker needles of course may be provided with a corresponding hub as the insulin in the device of course without problems may pass a thicker needle having a larger bore.
Although G30 needles cause less pain and allow even crystalline insulin to pass provided the crystals have no dimensions larger than 15 .mu.m, the use of these needles is not without problems even by injection of solutions. One of the problems is that a relatively high pressure has to be established in the cartridge from which the medicine shall be pressed out which again means that an excessive force has to be exerted on a manually operated injection button. This may make it difficult for users with weak fingers to perform the injection sufficiently rapidly. With the high pressure in the cartridge, which is mainly of the kind wherein a piston closes one end of an cylinder ampoule whereas the other is closed by a rubber membrane which may be penetrated by a back needle of a double pointed needle to provide communication from the content of the ampoule through the hollow needle to the injection point of this needle, elasticity of the cartridge parts, especially the rubber membrane and the piston, may cause dripping from the needle when this needle is drawn out from the tissue into which it has been inserted during the injection. This means that not the whole set dose is actually injected.
It is the object of the invention to provide a thin needle by which the advantages of the G30 needle are enhanced and/or the drawbacks of this needle are overcome.